It has spread to more leaves, but not in any logical manner. Still no sign of the kind of distortion that comes from viruses, although the damage to the leaves does make them look bumpier, and buds are forming normally, internode distance is normal, etc. It's also not primarily affecting new growth but seems to be spreading in a haphazard way.

It makes me think this is an atypical (or maybe typical for Melothria scabra--I wouldn't know, as I have no previous experience with this plant) presentation of one of the mildews?

I sprayed a copper fungicide on all the plants and will prune out affected leaves. I may try a dilute bleach spray in a couple of days if things continue to progress.